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Why is ,. (comma-dot) treated specially?

From: Artur Malabarba
Subject: Why is ,. (comma-dot) treated specially?
Date: Wed, 21 Jan 2015 21:58:20 -0200

I ran into a problem the other day where writing
    `(message ,.some-var)
unexpectedly split the .some-var symbol without evaluating anything:
    (message (\,\. some-var))

Meanwhile, adding a space after the comma:
    `(message , .some-var)
yielded the expected result:
    (message [value of .some-var])

Upon inspecting `lread.c', I see that indeed ",." is treated specially
(look for references to Qcomma_dot), similarly to how ",@" is special.
On the other hand, I can find no mention of this "comma-dot" anywhere
inside backquote.el.

Is this a feature that was planned but is not yet finished? Why is the
dot handled specially after a comma if we don't do anything with it?


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